In rechargeable alkaline cells, weight and portability are important considerations. It is also advantageous for rechargeable alkaline cells to have long operating lives without the necessity of periodic maintenance. Rechargeable alkaline cells are used in numerous consumer devices such as calculators, portable radios, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable alkaline cells can also be configured as larger cells that can be used, for example, in industrial, aerospace, and electric vehicle applications.
There are many known types of Ni based cells such as nickel cadmium ("NiCd"), nickel metal hydride ("Ni-MH"), nickel hydrogen, nickel zinc, and nickel iron cells. NiCd rechargeable alkaline cells are the most widely used although it appears that they will be replaced by Ni-MH cells. Compared to NiCd cells, Ni-MH cells made of synthetically engineered materials have superior performance parameters and contain no toxic elements.
Ni-MH cells utilize a negative electrode that is capable of the reversible electrochemical storage of hydrogen. Ni-MH cells usually employ a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in the alkaline electrolyte. Upon application of an electrical potential across a Ni-MH cell, the Ni-MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, as shown in equation (1): ##EQU1##
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron. The reactions that take place at the nickel hydroxide positive electrode of a Ni-MH cell are shown in equation (2): ##EQU2##
Ni-MH materials are discussed in detail in U.S. Pat. No. 5,277,999 to Ovshinsky, et al., the contents of which are incorporated by reference.
In alkaline rechargeable cells, the discharge capacity of a nickel based positive electrode is limited by the amount of active material, and the charging efficiencies. The charge capacities of a Cd negative electrode and a MH negative electrode are both provided in excess, to maintain the optimum capacity and provide overcharge protection. Thus, a goal in making the nickel positive electrode is to obtain as high an energy density as possible. The volume of a nickel hydroxide positive electrode is sometimes more important than weight. The volumetric capacity density is usually measured in mAh/cc and specific capacity is written as mAh/g.
At present, sintered or pasted nickel hydroxide positive electrodes are used in NiCd and Ni-MH cells. The process of making sintered electrodes is well known in the art. Conventional sintered electrodes normally have an energy density of around 480-500 mAh/cc. In order to achieve significantly higher capacity, the current trend has been away from sintered positive electrodes and toward foamed and pasted electrodes.
Sintered nickel electrodes have been the dominant nickel electrode technology for several decades for most applications. These consist of a porous nickel plaque of sintered high surface area nickel particles impregnated with nickel hydroxide active material either by chemical or electrochemical methods. While expensive, sintered electrodes provide high power, high reliability, and high cycle life, but not the highest energy density. They are likely to remain important for high reliability military and aerospace applications for some time.
Pasted nickel electrodes consist of nickel hydroxide particles in contact with a conductive network or substrate, preferably having a high surface area. There have been several variants of these electrodes including the so-called plastic-bonded nickel electrodes which utilize graphite as a microconductor and also including the so-called foam-metal electrodes which utilize high porosity nickel foam as a substrate loaded with spherical nickel hydroxide particles and cobalt conductivity enhancing additives. Pasted electrodes of the foam-metal type now dominate the consumer market due to their low cost, simple manufacturing, and higher energy density relative to sintered nickel electrodes.
Conventionally, the nickel battery electrode reaction has been considered to be a one electron process involving oxidation of divalent nickel hydroxide to trivalent nickel oxyhydroxide on charge and subsequent discharge of trivalent nickel oxyhydroxide to divalent nickel hydroxide, as shown in equation 2 hereinbelow.
Some recent evidence suggests that quadrivalent nickel is involved in the nickel hydroxide redox reaction. This is not a new concept. In fact, the existence of quadrivalent nickel was first proposed by Thomas Edison in some of his early battery patents. However, full utilization of quadrivalent nickel has never been investigated.
In practice, electrode capacity beyond the one-electron transfer theoretical capacity is not usually observed. One reason for this is incomplete utilization of the active material due to isolation of oxidized material. Because reduced nickel hydroxide material has a high resistance, the reduction of nickel hydroxide adjacent the current collector forms a less conductive surface that interferes with the subsequent reduction of oxidized active material that is farther away.
As discussed in U.S. Pat. No. 5,348,822, nickel hydroxide positive electrode material in its most basic form has a maximum theoretical specific capacity of 289 mAh/g, when one charge/discharge cycles from a .beta.II phase to a .beta.III phase and results in one electron transferred per nickel atom. It was recognized in the prior art that greater than one electron transfer could be realized by deviating from the .beta.II and .beta.III limitations and cycling between a highly oxidized .gamma.-phase nickel hydroxide phase and the .beta.II phase. However, it was also widely recognized that such gamma phase nickel hydroxide formation destroyed reversible structural stability and therefore cycle life was unacceptably degraded. A large number of patents and technical literature disclosed modifications to nickel hydroxide material designed to inhibit and/or prevent the destructive formation of the transition to the .gamma.-phase, even though the higher attainable capacity through the use of .gamma.-phase is lost.
Attempts to improve nickel hydroxide positive electrode materials began with the addition of modifiers to compensate for what was perceived as the inherent problems of the material. The use of compositions such as NiCoCd, NiCoZn, NiCoMg, and their analogues are described, for example, in the following patents:
U.S. Pat. No. Re. 34,752, to Oshitani, et al., reissued Oct. 4, 1994, describes a nickel hydroxide active material that contains nickel hydroxide containing 1-10 wt % zinc or 1-3 wt % magnesium to suppress the production of gamma-NiOOH. The invention is directed toward increasing utilization and discharge capacity of the positive electrode. Percent utilization and percent discharge capacity are discussed in the presence of various additives.
Oshitani, et al. describe the lengths that routineers in the art thought it was necessary to go to in order to inhibit .gamma.-NiOOH. The patent states:
Further, since the current density increased in accordance with the reduction of the specific surface area, a large amount of higher oxide .gamma.-NiOOH may be produced, which may cause fatal phenomena such as stepped discharge characteristics and/or swelling. The swelling due to the production of .gamma.-NiOOH in the nickel electrode is caused by the large change of the density from high density .beta.-NiOOH to low density .gamma.-NiOOH. The inventors have already found that the production of .gamma.-NiOOH can effectively be prevented by addition of a small amount of cadmium in a solid solution into the nickel hydroxide. However, it is desired to achieve the substantially same or more excellent effect by utilizing additive other than the cadmium from the viewpoint of the environmental pollution."
U.S. Pat. No. 5,366,831, to Watada, et al., issued Nov. 22, 1994, describes the addition of a single Group II element (such as Zn, Ba, and Co) in a solid solution with nickel hydroxide active material. The Group II element is described as preventing the formation of gamma phase nickel hydroxide thereby reducing swelling, and the cobalt is described as reducing the oxygen overvoltage thereby increasing high temperature charging efficiency. Both oxygen overvoltage and charge efficiency are described as increasing with increasing cobalt.
U.S. Pat. No. 5,451,475, to Ohta, et al., issued Sep. 19, 1995, describes the positive nickel hydroxide electrode material as fabricated with at least one of the following elements added to the surface of the particles thereof: cobalt, cobalt hydroxide, cobalt oxide, carbon powder, and at least one powdery compound of Ca, Sr, Ba, Cu, Ag, and Y. The cobalt, cobalt compound, and carbon are described as constituents of a conductive network to improve charging efficiency and conductivity. The powdery compound is described as adsorbed to the surface of the nickel hydroxide active material where it increases the overvoltage, for evolution of oxygen, thereby increasing nickel hydroxide utilization at high temperature. Ohta, et al. claims that increased utilization in NiMH cells using the disclosed invention remains constant up to a high number of charge/discharge cycles and utilization does not drop as much at higher temperatures as it does in cells that do not embody the invention.
U.S. Pat. No. 5,455,125 to Matsumoto, et al., issued Oct. 3, 1995, describes a battery having a positive electrode comprising nickel hydroxide pasted on a nickel foam substrate with solid solution regions of Co and salts of Cd, Zn, Ca, Ag, Mn, Sr, V, Ba, Sb, Y, and rare earth elements. The addition of the solid solution regions is intended to control the oxygen overvoltage during charging. The further external addition of "electric conducting agents" such as powdered cobalt, cobalt oxide, nickel, graphite, "and the like," is also described. Energy density is shown as constant at 72 Wh/kg at 20.degree. C. and 56 Wh/kg at 45.degree. C. for embodiments of the invention over the life of the NiMH cell.
U.S. Pat. No. 5,466,543, to Ikoma, et al., issued Nov. 14, 1995, describes batteries having improved nickel hydroxide utilization over a wide temperature range and increased oxygen overvoltage resulting from the incorporation of at least one compound of yttrium, indium, antimony, barium, or beryllium, and at least one compound of cobalt or calcium into the positive electrode. Cobalt hydroxide, calcium oxide, calcium hydroxide, calcium fluoride, calcium peroxide, and calcium silicate are specifically described compounds. Additionally described additives are cobalt, powdery carbon, and nickel. The specification particularly describes AA cells using a positive electrode containing 3 wt % zinc oxide and 3 wt % calcium hydroxide as superior in terms of cycle life (250 cycles at 0.degree. C., 370 cycles at 20.degree. C., and 360 cycles at 40.degree. C.) and discharge capacity (950 mAh at 20.degree. C., 850 mAh at 40.degree. C., and 780 mAh at 50.degree. C.).
U.S. Pat. No. 5,489,314, to Bodauchi, et al., issued Feb. 6, 1996, describes mixing the nickel hydroxide positive electrode material with a cobalt powder compound followed by an oxidation step to form a beta cobalt oxyhydroxide on the surface of the nickel hydroxide powder.
U.S. Pat. No. 5,506,070, to Mori, et al., issued Apr. 9, 1996, describes nickel hydroxide positive electrode material containing 2-8 wt % zinc mixed with 5-15% cobalt monoxide. The zinc reduces swelling and the cobalt increases utilization. The capacity of the resulting electrode is stated as being "improved up to 600 mAh/cc" without further description.
U.S. Pat. No. 5,571,636, to Ohta, et al., issued Nov. 5, 1996, describes the addition of at least one powdery compound of Ca, Sr, Ba, Cu, Ag, and Y to the surface of nickel hydroxide active positive electrode material. This patent states that these compounds are adsorbed to the surface of the nickel hydroxide active material creating a conductive network that increases the oxygen overvoltage and improves utilization of the active material at high temperatures. Increased utilization in NiMH cells using the '636 invention remains constant up to a large number of cycles and utilization does not drop as much at higher temperatures as it does in cells that do not embody the invention.
In all of the prior art, the basic nickel hydroxide material is treated, most commonly, by the addition of a single element, usually Co compounds, to increase electrical conductivity and usually one other element, usually Cd or Zn, to suppress and/or prevent .gamma.-phase formation. The mechanisms for the asserted improvements in all the above patents are attributable to the following effects:
1. Improved speed of activation, resistance to poisons, and marginal capacity improvement via increased utilization. At the present time, most commercial nickel metal hydride batteries achieve these effects through the external addition of up to 5 wt % cobalt and/or cobalt-containing compound. It is generally believed that the major reason cobalt is effective at these levels is because it creates an extensive external conductive network independent of the nickel hydroxide material. Frequently, powdered carbon, powdered cobalt metal, and powdered nickel metal are also added to create separate conductive networks and thereby improve utilization. Of course, a major drawback of increasing the amount of such additives is that the amount of active nickel hydroxide electrode material is correspondingly reduced, thereby reducing capacity. Further, since Co is expensive, the addition of even minimum amounts of Co greatly increases cost.
2. Cycle life is extended by decreasing swelling that is initiated by density changes between the oxidized and reduced states of the nickel hydroxide material. Swelling, in turn, is accelerated by the uncontrolled density changes between .beta.II-.beta.III phase nickel hydroxide and .alpha.-.gamma. or .beta.II-.gamma. phase nickel hydroxide. Cd and Zn incorporated into the nickel hydroxide effectively reduce the swelling by reducing the difference in density in the charged and discharged material and increasing the mechanical stability of the nickel hydroxide material itself. This is accomplished by promoting oxygen evolution and thereby reducing charge acceptance which prevents the nickel hydroxide material from attaining the highly oxidized state (the .gamma.-phase state). However, by suppressing or at least significantly inhibiting .gamma.-phase formation, the nickel hydroxide is limited to transferring no more than one electron per Ni atom. Further, in order to effectively inhibit .gamma.-phase nickel hydroxide, it is necessary to employ a relatively high wt % of the inhibitor element such as Zn or Cd, which high percentage results in a greatly reduced amount of active material being present thereby resulting in reduced electrochemical capacity.
3. The aforementioned "safety release" mechanism of oxygen evolution to avoid highly oxidized states of nickel hydroxide material actually is an impediment to high temperature operation because a significant increase in the rate of oxygen evolution occurs with increasing temperature. The effect of such increased oxygen evolution is a very substantial decrease in utilization and ultimately a reduction in energy storage at higher temperatures in the NiMH battery using these materials. At 55.degree. C., for example, run times of a battery may be reduced by 35-55% compared to the room temperature performance of that battery.
Elevated operational temperature conditions aside, none of these prior art modifications result in more than an incremental improvement in performance and none result in a significant increase in the capacity of the nickel hydroxide material itself, even at room temperature. Further, these modifications fail to address the special operational requirements of NiMH batteries, particularly when NiMH batteries are used in electric vehicles, hybrid vehicles, scooters and other high capacity, high drain rate applications. Because NiMH negative electrodes have been improved and now exhibit an extremely high storage capacity, the nickel hydroxide positive electrode material is essentially the limiting factor in overall battery capacity. This makes improving the electrochemical performance of the nickel hydroxide material in all areas more important than in the past. Unfortunately, the elements currently added to the nickel hydroxide material result in insufficient improvements in performance before competing deleterious mechanisms and effects occur. For example, Cd cannot be used in any commercial battery because of the environmental impact thereof, and Co and Zn appear to become most effective only at levels that result in a significant decrease in cell capacity; more specifically, energy per electrode weight.
Ovshinsky and his team have developed positive electrode materials that have demonstrated reliable transfer of more than one electron per nickel atom. Such materials are described in U.S. Pat. Nos. 5,344,728 and 5,348,822 (which describe stabilized disordered positive electrode materials) and copending U.S. patent application Ser. No. 08/300,610 filed Aug. 23, 1994, and U.S. patent application Ser. No. 08/308,764 filed Sep. 19, 1994.
Previously all of the work on nickel hydroxide positive electrode material has concentrated on improving it's conductivity in two ways. First electrically conductive additives have been externally mixed with the nickel hydroxide materials used to produce pasted electrodes. Such additives include Co, CoO, Ni, Cu, and C. The additives are generally in the form of powder, fibers or the like. These techniques have achieved moderate success in that Ni-MH batteries have achieved impressive gains in high rate discharge performance. However, there are two remaining problems. First, the emergence of hybrid electric vehicles has demanded that Ni-MH batteries achieve 1000 W/kg of power. Conventional electric vehicle batteries achieve 250 W/kg and special designs achieve 500-600 W/kg. Second, even present power levels are achieved by a very expensive and elaborate positive electrode embodiment (i.e., an expensive foam metal skeleton and expensive use of cobalt compounds).
The second way in which artisans have increased the conductivity of nickel hydroxide is by co-precipitating cobalt hydroxide along with nickel hydroxide to increase it's internal conductivity. While NiCo co-precipitates have better conductivity and utilization than pure nickel hydroxide, the improvement can only be considered incremental with no room for further improvement.
The two methods discussed above, while increasing the power and capacity of the nickel hydroxide materials and electrodes have still not realized the full potential thereof. As stated above, there is still a need for significant gains in power and high rate discharge capability. Therefore, there is a need in the art for additional improvements in the conductivity of positive electrode materials and, specifically, in the conductivity of nickel hydroxide for use in rechargeable battery applications.